it would be as easy as putting a slider to reduce damage taken/increase damage inflicted
That would compromise the dev's vision.
Celeste is about overcoming your personal struggle. Dark souls is about overcoming (or failing to overcome) the struggle forced on you by an uncaring world.
Nobody's ever said it would be impossible, mechanically, to add an easy mode. The problem is that it flies in the face of the message of the game.
Of course, I've no clue how this slider would interact with multiplayer - an entire part of the game that people tend to forget in conversations like this.
The photo was most likely taken from the outside of the fence, as evidenced by the smaller picket fence in the foreground